/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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 *
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 *
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 *
 */

package java.util;

import java.util.function.Consumer;

/**
 * Doubly-linked list implementation of the {@code List} and {@code Deque}
 * interfaces.  Implements all optional list operations, and permits all
 * elements (including {@code null}).
 *
 * <p>All of the operations perform as could be expected for a doubly-linked
 * list.  Operations that index into the list will traverse the list from
 * the beginning or the end, whichever is closer to the specified index.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a linked list concurrently, and at least
 * one of the threads modifies the list structurally, it <i>must</i> be
 * synchronized externally.  (A structural modification is any operation
 * that adds or deletes one or more elements; merely setting the value of
 * an element is not a structural modification.)  This is typically
 * accomplished by synchronizing on some object that naturally
 * encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 *   List list = Collections.synchronizedList(new LinkedList(...));</pre>
 *
 * <p>The iterators returned by this class's {@code iterator} and
 * {@code listIterator} methods are <i>fail-fast</i>: if the list is
 * structurally modified at any time after the iterator is created, in
 * any way except through the Iterator's own {@code remove} or
 * {@code add} methods, the iterator will throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than
 * risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this collection
 * @author Josh Bloch
 * @see List
 * @see ArrayList
 * @since 1.2
 * /// 底层的数据结构, CRUD的时间复杂度
 * /// 如何实现ConcurrentModificationException的检查
 * /// 如何确保一次next只能调用一次remove
 * /// 如何保证可边iterate边删除
 * /// 最多可放多少元素
 */

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable {

  transient int size = 0;

  /**
   * Pointer to first node.
   * Invariant: (first == null && last == null) ||
   * (first.prev == null && first.item != null)
   */
  transient Node<E> first;

  /**
   * Pointer to last node.
   * Invariant: (first == null && last == null) ||
   * (last.next == null && last.item != null)
   */
  transient Node<E> last;

  /**
   * Constructs an empty list.
   */
  public LinkedList() {
  }

  /**
   * Constructs a list containing the elements of the specified
   * collection, in the order they are returned by the collection's
   * iterator.
   *
   * @param c the collection whose elements are to be placed into this list
   * @throws NullPointerException if the specified collection is null
   */
  public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
  }

  /**
   * Links e as first element.
   */
  private void linkFirst(E e) {
    final Node<E> f = first;
    final Node<E> newNode = new Node<>(null, e, f);
    first = newNode;
    if (f == null) {
      last = newNode;
    } else {
      f.prev = newNode;
    }
    size++;
    modCount++;
  }

  /**
   * Links e as last element.
   */
  void linkLast(E e) {
    final Node<E> l = last;
    final Node<E> newNode = new Node<>(l, e, null);
    last = newNode;
    if (l == null) {
      first = newNode;
    } else {
      l.next = newNode;
    }
    size++;
    modCount++;
  }

  /**
   * Inserts element e before non-null Node succ.
   */
  void linkBefore(E e, Node<E> succ) {
    // assert succ != null;
    final Node<E> pred = succ.prev;
    final Node<E> newNode = new Node<>(pred, e, succ);
    succ.prev = newNode;
    if (pred == null) {
      first = newNode;
    } else {
      pred.next = newNode;
    }
    size++;
    modCount++;
  }

  /**
   * Unlinks non-null first node f.
   */
  private E unlinkFirst(Node<E> f) {
    // assert f == first && f != null;
    final E element = f.item;
    final Node<E> next = f.next;
    f.item = null;
    f.next = null; // help GC
    first = next;
    if (next == null) {
      last = null;
    } else {
      next.prev = null;
    }
    size--;
    modCount++;
    return element;
  }

  /**
   * Unlinks non-null last node l.
   */
  private E unlinkLast(Node<E> l) {
    // assert l == last && l != null;
    final E element = l.item;
    final Node<E> prev = l.prev;
    l.item = null;
    l.prev = null; // help GC
    last = prev;
    if (prev == null) {
      first = null;
    } else {
      prev.next = null;
    }
    size--;
    modCount++;
    return element;
  }

  /**
   * Unlinks non-null node x.
   */
  E unlink(Node<E> x) {
    // assert x != null;
    final E element = x.item;
    final Node<E> next = x.next;
    final Node<E> prev = x.prev;

    if (prev == null) {
      first = next;
    } else {
      prev.next = next;
      x.prev = null;
    }

    if (next == null) {
      last = prev;
    } else {
      next.prev = prev;
      x.next = null;
    }

    x.item = null;
    size--;
    modCount++;
    return element;
  }

  /**
   * Returns the first element in this list.
   *
   * @return the first element in this list
   * @throws NoSuchElementException if this list is empty
   */
  public E getFirst() {
    final Node<E> f = first;
    if (f == null) {
      throw new NoSuchElementException();
    }
    return f.item;
  }

  /**
   * Returns the last element in this list.
   *
   * @return the last element in this list
   * @throws NoSuchElementException if this list is empty
   */
  public E getLast() {
    final Node<E> l = last;
    if (l == null) {
      throw new NoSuchElementException();
    }
    return l.item;
  }

  /**
   * Removes and returns the first element from this list.
   *
   * @return the first element from this list
   * @throws NoSuchElementException if this list is empty
   */
  public E removeFirst() {
    final Node<E> f = first;
    if (f == null) {
      throw new NoSuchElementException();
    }
    return unlinkFirst(f);
  }

  /**
   * Removes and returns the last element from this list.
   *
   * @return the last element from this list
   * @throws NoSuchElementException if this list is empty
   */
  public E removeLast() {
    final Node<E> l = last;
    if (l == null) {
      throw new NoSuchElementException();
    }
    return unlinkLast(l);
  }

  /**
   * Inserts the specified element at the beginning of this list.
   *
   * @param e the element to add
   */
  public void addFirst(E e) {
    linkFirst(e);
  }

  /**
   * Appends the specified element to the end of this list.
   *
   * <p>This method is equivalent to {@link #add}.
   *
   * @param e the element to add
   */
  public void addLast(E e) {
    linkLast(e);
  }

  /**
   * Returns {@code true} if this list contains the specified element.
   * More formally, returns {@code true} if and only if this list contains
   * at least one element {@code e} such that
   * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
   *
   * @param o element whose presence in this list is to be tested
   * @return {@code true} if this list contains the specified element
   */
  public boolean contains(Object o) {
    return indexOf(o) != -1;
  }

  /**
   * Returns the number of elements in this list.
   *
   * @return the number of elements in this list
   */
  public int size() {
    return size;
  }

  /**
   * Appends the specified element to the end of this list.
   *
   * <p>This method is equivalent to {@link #addLast}.
   *
   * @param e element to be appended to this list
   * @return {@code true} (as specified by {@link Collection#add})
   */
  public boolean add(E e) {
    linkLast(e);
    return true;
  }

  /**
   * Removes the first occurrence of the specified element from this list,
   * if it is present.  If this list does not contain the element, it is
   * unchanged.  More formally, removes the element with the lowest index
   * {@code i} such that
   * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
   * (if such an element exists).  Returns {@code true} if this list
   * contained the specified element (or equivalently, if this list
   * changed as a result of the call).
   *
   * @param o element to be removed from this list, if present
   * @return {@code true} if this list contained the specified element
   */
  /// 注意只去除第一个,为什么不提供removeAll?
  public boolean remove(Object o) {
    if (o == null) {
      for (Node<E> x = first; x != null; x = x.next) {
        if (x.item == null) {
          unlink(x);
          return true;
        }
      }
    } else {
      for (Node<E> x = first; x != null; x = x.next) {
        if (o.equals(x.item)) {
          unlink(x);
          return true;
        }
      }
    }
    return false;
  }

  /**
   * Appends all of the elements in the specified collection to the end of
   * this list, in the order that they are returned by the specified
   * collection's iterator.  The behavior of this operation is undefined if
   * the specified collection is modified while the operation is in
   * progress.  (Note that this will occur if the specified collection is
   * this list, and it's nonempty.)
   *
   * @param c collection containing elements to be added to this list
   * @return {@code true} if this list changed as a result of the call
   * @throws NullPointerException if the specified collection is null
   */
  public boolean addAll(Collection<? extends E> c) {
    return addAll(size, c);
  }

  /**
   * Inserts all of the elements in the specified collection into this
   * list, starting at the specified position.  Shifts the element
   * currently at that position (if any) and any subsequent elements to
   * the right (increases their indices).  The new elements will appear
   * in the list in the order that they are returned by the
   * specified collection's iterator.
   *
   * @param index index at which to insert the first element from the specified collection
   * @param c collection containing elements to be added to this list
   * @return {@code true} if this list changed as a result of the call
   * @throws IndexOutOfBoundsException {@inheritDoc}
   * @throws NullPointerException if the specified collection is null
   */
  public boolean addAll(int index, Collection<? extends E> c) {
    checkPositionIndex(index);

    Object[] a = c.toArray();
    int numNew = a.length;
    if (numNew == 0) {
      return false;
    }

    Node<E> pred, succ;
    if (index == size) {
      succ = null;
      pred = last;
    } else {
      succ = node(index);
      pred = succ.prev;
    }

    for (Object o : a) {
      @SuppressWarnings("unchecked") E e = (E) o;
      Node<E> newNode = new Node<>(pred, e, null);
      if (pred == null) {
        first = newNode;
      } else {
        pred.next = newNode;
      }
      pred = newNode;
    }

    if (succ == null) {
      last = pred;
    } else {
      pred.next = succ;
      succ.prev = pred;
    }

    size += numNew;
    /// addAll只记录一次mod count!
    modCount++;
    return true;
  }

  /**
   * Removes all of the elements from this list.
   * The list will be empty after this call returns.
   */
  public void clear() {
    // Clearing all of the links between nodes is "unnecessary", but:
    // - helps a generational GC if the discarded nodes inhabit
    //   more than one generation
    // - is sure to free memory even if there is a reachable Iterator
    for (Node<E> x = first; x != null; ) {
      Node<E> next = x.next;
      x.item = null;
      x.next = null;
      x.prev = null;
      x = next;
    }
    first = last = null;
    size = 0;
    modCount++;
  }

  // Positional Access Operations

  /**
   * Returns the element at the specified position in this list.
   *
   * @param index index of the element to return
   * @return the element at the specified position in this list
   * @throws IndexOutOfBoundsException {@inheritDoc}
   */
  public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
  }

  /**
   * Replaces the element at the specified position in this list with the
   * specified element.
   *
   * @param index index of the element to replace
   * @param element element to be stored at the specified position
   * @return the element previously at the specified position
   * @throws IndexOutOfBoundsException {@inheritDoc}
   */
  public E set(int index, E element) {
    checkElementIndex(index);
    Node<E> x = node(index);
    E oldVal = x.item;
    x.item = element;
    return oldVal;
  }

  /**
   * Inserts the specified element at the specified position in this list.
   * Shifts the element currently at that position (if any) and any
   * subsequent elements to the right (adds one to their indices).
   *
   * @param index index at which the specified element is to be inserted
   * @param element element to be inserted
   * @throws IndexOutOfBoundsException {@inheritDoc}
   */
  public void add(int index, E element) {
    checkPositionIndex(index);

    if (index == size) {
      linkLast(element);
    } else {
      linkBefore(element, node(index));
    }
  }

  /**
   * Removes the element at the specified position in this list.  Shifts any
   * subsequent elements to the left (subtracts one from their indices).
   * Returns the element that was removed from the list.
   *
   * @param index the index of the element to be removed
   * @return the element previously at the specified position
   * @throws IndexOutOfBoundsException {@inheritDoc}
   */
  public E remove(int index) {
    checkElementIndex(index);
    return unlink(node(index));
  }

  /**
   * Tells if the argument is the index of an existing element.
   */
  private boolean isElementIndex(int index) {
    return index >= 0 && index < size;
  }

  /**
   * Tells if the argument is the index of a valid position for an
   * iterator or an add operation.
   */
  private boolean isPositionIndex(int index) {
    return index >= 0 && index <= size;
  }

  /**
   * Constructs an IndexOutOfBoundsException detail message.
   * Of the many possible refactorings of the error handling code,
   * this "outlining" performs best with both server and client VMs.
   */
  private String outOfBoundsMsg(int index) {
    return "Index: " + index + ", Size: " + size;
  }

  private void checkElementIndex(int index) {
    if (!isElementIndex(index)) {
      throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
  }

  private void checkPositionIndex(int index) {
    if (!isPositionIndex(index)) {
      throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
  }

  /**
   * Returns the (non-null) Node at the specified element index.
   */
  Node<E> node(int index) {
    // assert isElementIndex(index);

    if (index < (size >> 1)) {
      Node<E> x = first;
      for (int i = 0; i < index; i++) {
        x = x.next;
      }
      return x;
    } else {
      Node<E> x = last;
      for (int i = size - 1; i > index; i--) {
        x = x.prev;
      }
      return x;
    }
  }

  // Search Operations

  /**
   * Returns the index of the first occurrence of the specified element
   * in this list, or -1 if this list does not contain the element.
   * More formally, returns the lowest index {@code i} such that
   * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
   * or -1 if there is no such index.
   *
   * @param o element to search for
   * @return the index of the first occurrence of the specified element in this list, or -1 if this
   * list does not contain the element
   */
  /// O(n)的复杂度，所以list的contains也不会快
  public int indexOf(Object o) {
    int index = 0;
    if (o == null) {
      for (Node<E> x = first; x != null; x = x.next) {
        if (x.item == null) {
          return index;
        }
        index++;
      }
    } else {
      for (Node<E> x = first; x != null; x = x.next) {
        if (o.equals(x.item)) {
          return index;
        }
        index++;
      }
    }
    return -1;
  }

  /**
   * Returns the index of the last occurrence of the specified element
   * in this list, or -1 if this list does not contain the element.
   * More formally, returns the highest index {@code i} such that
   * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
   * or -1 if there is no such index.
   *
   * @param o element to search for
   * @return the index of the last occurrence of the specified element in this list, or -1 if this
   * list does not contain the element
   */
  public int lastIndexOf(Object o) {
    int index = size;
    if (o == null) {
      for (Node<E> x = last; x != null; x = x.prev) {
        index--;
        if (x.item == null) {
          return index;
        }
      }
    } else {
      for (Node<E> x = last; x != null; x = x.prev) {
        index--;
        if (o.equals(x.item)) {
          return index;
        }
      }
    }
    return -1;
  }

  // Queue operations.

  /**
   * Retrieves, but does not remove, the head (first element) of this list.
   *
   * @return the head of this list, or {@code null} if this list is empty
   * @since 1.5
   */
  public E peek() {
    final Node<E> f = first;
    return (f == null) ? null : f.item;
  }

  /**
   * Retrieves, but does not remove, the head (first element) of this list.
   *
   * @return the head of this list
   * @throws NoSuchElementException if this list is empty
   * @since 1.5
   */
  public E element() {
    return getFirst();
  }

  /**
   * Retrieves and removes the head (first element) of this list.
   *
   * @return the head of this list, or {@code null} if this list is empty
   * @since 1.5
   */
  public E poll() {
    final Node<E> f = first;
    return (f == null) ? null : unlinkFirst(f);
  }

  /**
   * Retrieves and removes the head (first element) of this list.
   *
   * @return the head of this list
   * @throws NoSuchElementException if this list is empty
   * @since 1.5
   */
  public E remove() {
    return removeFirst();
  }

  /**
   * Adds the specified element as the tail (last element) of this list.
   *
   * @param e the element to add
   * @return {@code true} (as specified by {@link Queue#offer})
   * @since 1.5
   */
  public boolean offer(E e) {
    return add(e);
  }

  // Deque operations

  /**
   * Inserts the specified element at the front of this list.
   *
   * @param e the element to insert
   * @return {@code true} (as specified by {@link Deque#offerFirst})
   * @since 1.6
   */
  public boolean offerFirst(E e) {
    addFirst(e);
    return true;
  }

  /**
   * Inserts the specified element at the end of this list.
   *
   * @param e the element to insert
   * @return {@code true} (as specified by {@link Deque#offerLast})
   * @since 1.6
   */
  public boolean offerLast(E e) {
    addLast(e);
    return true;
  }

  /**
   * Retrieves, but does not remove, the first element of this list,
   * or returns {@code null} if this list is empty.
   *
   * @return the first element of this list, or {@code null} if this list is empty
   * @since 1.6
   */
  public E peekFirst() {
    final Node<E> f = first;
    return (f == null) ? null : f.item;
  }

  /**
   * Retrieves, but does not remove, the last element of this list,
   * or returns {@code null} if this list is empty.
   *
   * @return the last element of this list, or {@code null} if this list is empty
   * @since 1.6
   */
  public E peekLast() {
    final Node<E> l = last;
    return (l == null) ? null : l.item;
  }

  /**
   * Retrieves and removes the first element of this list,
   * or returns {@code null} if this list is empty.
   *
   * @return the first element of this list, or {@code null} if this list is empty
   * @since 1.6
   */
  public E pollFirst() {
    final Node<E> f = first;
    return (f == null) ? null : unlinkFirst(f);
  }

  /**
   * Retrieves and removes the last element of this list,
   * or returns {@code null} if this list is empty.
   *
   * @return the last element of this list, or {@code null} if this list is empty
   * @since 1.6
   */
  public E pollLast() {
    final Node<E> l = last;
    return (l == null) ? null : unlinkLast(l);
  }

  /**
   * Pushes an element onto the stack represented by this list.  In other
   * words, inserts the element at the front of this list.
   *
   * <p>This method is equivalent to {@link #addFirst}.
   *
   * @param e the element to push
   * @since 1.6
   */
  public void push(E e) {
    addFirst(e);
  }

  /**
   * Pops an element from the stack represented by this list.  In other
   * words, removes and returns the first element of this list.
   *
   * <p>This method is equivalent to {@link #removeFirst()}.
   *
   * @return the element at the front of this list (which is the top of the stack represented by
   * this list)
   * @throws NoSuchElementException if this list is empty
   * @since 1.6
   */
  public E pop() {
    return removeFirst();
  }

  /**
   * Removes the first occurrence of the specified element in this
   * list (when traversing the list from head to tail).  If the list
   * does not contain the element, it is unchanged.
   *
   * @param o element to be removed from this list, if present
   * @return {@code true} if the list contained the specified element
   * @since 1.6
   */
  public boolean removeFirstOccurrence(Object o) {
    return remove(o);
  }

  /**
   * Removes the last occurrence of the specified element in this
   * list (when traversing the list from head to tail).  If the list
   * does not contain the element, it is unchanged.
   *
   * @param o element to be removed from this list, if present
   * @return {@code true} if the list contained the specified element
   * @since 1.6
   */
  public boolean removeLastOccurrence(Object o) {
    if (o == null) {
      for (Node<E> x = last; x != null; x = x.prev) {
        if (x.item == null) {
          unlink(x);
          return true;
        }
      }
    } else {
      for (Node<E> x = last; x != null; x = x.prev) {
        if (o.equals(x.item)) {
          unlink(x);
          return true;
        }
      }
    }
    return false;
  }

  /**
   * Returns a list-iterator of the elements in this list (in proper
   * sequence), starting at the specified position in the list.
   * Obeys the general contract of {@code List.listIterator(int)}.<p>
   *
   * The list-iterator is <i>fail-fast</i>: if the list is structurally
   * modified at any time after the Iterator is created, in any way except
   * through the list-iterator's own {@code remove} or {@code add}
   * methods, the list-iterator will throw a
   * {@code ConcurrentModificationException}.  Thus, in the face of
   * concurrent modification, the iterator fails quickly and cleanly, rather
   * than risking arbitrary, non-deterministic behavior at an undetermined
   * time in the future.
   *
   * @param index index of the first element to be returned from the list-iterator (by a call to
   * {@code next})
   * @return a ListIterator of the elements in this list (in proper sequence), starting at the
   * specified position in the list
   * @throws IndexOutOfBoundsException {@inheritDoc}
   * @see List#listIterator(int)
   */
  public ListIterator<E> listIterator(int index) {
    checkPositionIndex(index);
    return new ListItr(index);
  }

  private class ListItr implements ListIterator<E> {

    private Node<E> lastReturned;
    private Node<E> next;
    private int nextIndex;
    private int expectedModCount = modCount;

    ListItr(int index) {
      // assert isPositionIndex(index);
      next = (index == size) ? null : node(index);
      nextIndex = index;
    }

    public boolean hasNext() {
      return nextIndex < size;
    }

    public E next() {
      checkForComodification();
      if (!hasNext()) {
        throw new NoSuchElementException();
      }

      lastReturned = next;
      next = next.next;
      nextIndex++;
      return lastReturned.item;
    }

    public boolean hasPrevious() {
      return nextIndex > 0;
    }

    public E previous() {
      checkForComodification();
      if (!hasPrevious()) {
        throw new NoSuchElementException();
      }

      lastReturned = next = (next == null) ? last : next.prev;
      nextIndex--;
      return lastReturned.item;
    }

    public int nextIndex() {
      return nextIndex;
    }

    public int previousIndex() {
      return nextIndex - 1;
    }

    /// 每次调用next时更新lastReturned，每次调remove后再置为null，这样很方便实现一次next只能调用一次remove
    public void remove() {
      checkForComodification();
      if (lastReturned == null) {
        throw new IllegalStateException();
      }

      Node<E> lastNext = lastReturned.next;
      unlink(lastReturned);
      if (next == lastReturned) {
        next = lastNext;
      } else {
        nextIndex--;
      }
      lastReturned = null;
      /// addd 和remove时维护iterator视角的mod count
      expectedModCount++;
    }

    public void set(E e) {
      if (lastReturned == null) {
        throw new IllegalStateException();
      }
      checkForComodification();
      lastReturned.item = e;
    }

    public void add(E e) {
      checkForComodification();
      lastReturned = null;
      if (next == null) {
        linkLast(e);
      } else {
        linkBefore(e, next);
      }
      nextIndex++;
      expectedModCount++;
    }

    public void forEachRemaining(Consumer<? super E> action) {
      Objects.requireNonNull(action);
      while (modCount == expectedModCount && nextIndex < size) {
        action.accept(next.item);
        lastReturned = next;
        next = next.next;
        nextIndex++;
      }
      checkForComodification();
    }

    final void checkForComodification() {
      /// 对比list视角和iterator视角的mod count
      if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
      }
    }
  }

  private static class Node<E> {

    E item;
    Node<E> next;
    Node<E> prev;

    Node(Node<E> prev, E element, Node<E> next) {
      this.item = element;
      this.next = next;
      this.prev = prev;
    }
  }

  /**
   * @since 1.6
   */
  public Iterator<E> descendingIterator() {
    return new DescendingIterator();
  }

  /**
   * Adapter to provide descending iterators via ListItr.previous
   */
  private class DescendingIterator implements Iterator<E> {

    private final ListItr itr = new ListItr(size());

    /// 很简单就实现了反向遍历
    public boolean hasNext() {
      return itr.hasPrevious();
    }

    public E next() {
      return itr.previous();
    }

    public void remove() {
      itr.remove();
    }
  }

  @SuppressWarnings("unchecked")
  private LinkedList<E> superClone() {
    try {
      return (LinkedList<E>) super.clone();
    } catch (CloneNotSupportedException e) {
      throw new InternalError(e);
    }
  }

  /**
   * Returns a shallow copy of this {@code LinkedList}. (The elements
   * themselves are not cloned.)
   *
   * @return a shallow copy of this {@code LinkedList} instance
   */
  public Object clone() {
    LinkedList<E> clone = superClone();

    // Put clone into "virgin" state
    clone.first = clone.last = null;
    clone.size = 0;
    clone.modCount = 0;

    // Initialize clone with our elements
    for (Node<E> x = first; x != null; x = x.next) {
      clone.add(x.item);
    }

    return clone;
  }

  /**
   * Returns an array containing all of the elements in this list
   * in proper sequence (from first to last element).
   *
   * <p>The returned array will be "safe" in that no references to it are
   * maintained by this list.  (In other words, this method must allocate
   * a new array).  The caller is thus free to modify the returned array.
   *
   * <p>This method acts as bridge between array-based and collection-based
   * APIs.
   *
   * @return an array containing all of the elements in this list in proper sequence
   */
  public Object[] toArray() {
    Object[] result = new Object[size];
    int i = 0;
    for (Node<E> x = first; x != null; x = x.next) {
      result[i++] = x.item;
    }
    return result;
  }

  /**
   * Returns an array containing all of the elements in this list in
   * proper sequence (from first to last element); the runtime type of
   * the returned array is that of the specified array.  If the list fits
   * in the specified array, it is returned therein.  Otherwise, a new
   * array is allocated with the runtime type of the specified array and
   * the size of this list.
   *
   * <p>If the list fits in the specified array with room to spare (i.e.,
   * the array has more elements than the list), the element in the array
   * immediately following the end of the list is set to {@code null}.
   * (This is useful in determining the length of the list <i>only</i> if
   * the caller knows that the list does not contain any null elements.)
   *
   * <p>Like the {@link #toArray()} method, this method acts as bridge between
   * array-based and collection-based APIs.  Further, this method allows
   * precise control over the runtime type of the output array, and may,
   * under certain circumstances, be used to save allocation costs.
   *
   * <p>Suppose {@code x} is a list known to contain only strings.
   * The following code can be used to dump the list into a newly
   * allocated array of {@code String}:
   *
   * <pre>
   *     String[] y = x.toArray(new String[0]);</pre>
   *
   * Note that {@code toArray(new Object[0])} is identical in function to
   * {@code toArray()}.
   *
   * @param a the array into which the elements of the list are to be stored, if it is big enough;
   * otherwise, a new array of the same runtime type is allocated for this purpose.
   * @return an array containing the elements of the list
   * @throws ArrayStoreException if the runtime type of the specified array is not a supertype of
   * the runtime type of every element in this list
   * @throws NullPointerException if the specified array is null
   */
  @SuppressWarnings("unchecked")
  public <T> T[] toArray(T[] a) {
    if (a.length < size) {
      a = (T[]) java.lang.reflect.Array.newInstance(
          a.getClass().getComponentType(), size);
    }
    int i = 0;
    Object[] result = a;
    for (Node<E> x = first; x != null; x = x.next) {
      result[i++] = x.item;
    }

    if (a.length > size) {
      a[size] = null;
    }

    return a;
  }

  private static final long serialVersionUID = 876323262645176354L;

  /**
   * Saves the state of this {@code LinkedList} instance to a stream
   * (that is, serializes it).
   *
   * @serialData The size of the list (the number of elements it contains) is emitted (int),
   * followed by all of its elements (each an Object) in the proper order.
   */
  private void writeObject(java.io.ObjectOutputStream s)
      throws java.io.IOException {
    // Write out any hidden serialization magic
    s.defaultWriteObject();

    // Write out size
    s.writeInt(size);

    // Write out all elements in the proper order.
    for (Node<E> x = first; x != null; x = x.next) {
      s.writeObject(x.item);
    }
  }

  /**
   * Reconstitutes this {@code LinkedList} instance from a stream
   * (that is, deserializes it).
   */
  @SuppressWarnings("unchecked")
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    // Read in any hidden serialization magic
    s.defaultReadObject();

    // Read in size
    int size = s.readInt();

    // Read in all elements in the proper order.
    for (int i = 0; i < size; i++) {
      linkLast((E) s.readObject());
    }
  }

  /**
   * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
   * and <em>fail-fast</em> {@link Spliterator} over the elements in this
   * list.
   *
   * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
   * {@link Spliterator#ORDERED}.  Overriding implementations should document
   * the reporting of additional characteristic values.
   *
   * @return a {@code Spliterator} over the elements in this list
   * @implNote The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED} and
   * implements {@code trySplit} to permit limited parallelism..
   * @since 1.8
   */
  @Override
  public Spliterator<E> spliterator() {
    return new LLSpliterator<E>(this, -1, 0);
  }

  /**
   * A customized variant of Spliterators.IteratorSpliterator
   */
  static final class LLSpliterator<E> implements Spliterator<E> {

    static final int BATCH_UNIT = 1 << 10;  // batch array size increment
    static final int MAX_BATCH = 1 << 25;  // max batch array size;
    final LinkedList<E> list; // null OK unless traversed
    Node<E> current;      // current node; null until initialized
    int est;              // size estimate; -1 until first needed
    int expectedModCount; // initialized when est set
    int batch;            // batch size for splits

    LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
      this.list = list;
      this.est = est;
      this.expectedModCount = expectedModCount;
    }

    final int getEst() {
      int s; // force initialization
      final LinkedList<E> lst;
      if ((s = est) < 0) {
        if ((lst = list) == null) {
          s = est = 0;
        } else {
          expectedModCount = lst.modCount;
          current = lst.first;
          s = est = lst.size;
        }
      }
      return s;
    }

    public long estimateSize() {
      return (long) getEst();
    }

    public Spliterator<E> trySplit() {
      Node<E> p;
      int s = getEst();
      if (s > 1 && (p = current) != null) {
        int n = batch + BATCH_UNIT;
        if (n > s) {
          n = s;
        }
        if (n > MAX_BATCH) {
          n = MAX_BATCH;
        }
        Object[] a = new Object[n];
        int j = 0;
        do {
          a[j++] = p.item;
        } while ((p = p.next) != null && j < n);
        current = p;
        batch = j;
        est = s - j;
        return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
      }
      return null;
    }

    public void forEachRemaining(Consumer<? super E> action) {
      Node<E> p;
      int n;
      if (action == null) {
        throw new NullPointerException();
      }
      if ((n = getEst()) > 0 && (p = current) != null) {
        current = null;
        est = 0;
        do {
          E e = p.item;
          p = p.next;
          action.accept(e);
        } while (p != null && --n > 0);
      }
      if (list.modCount != expectedModCount) {
        throw new ConcurrentModificationException();
      }
    }

    public boolean tryAdvance(Consumer<? super E> action) {
      Node<E> p;
      if (action == null) {
        throw new NullPointerException();
      }
      if (getEst() > 0 && (p = current) != null) {
        --est;
        E e = p.item;
        current = p.next;
        action.accept(e);
        if (list.modCount != expectedModCount) {
          throw new ConcurrentModificationException();
        }
        return true;
      }
      return false;
    }

    public int characteristics() {
      return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
    }
  }

}
